A retreaded tire comprised of a tread and carcass with an intervening rubber layer, conventionally referred to as a cushion, wherein the tread has an oil rich rubber-based adhesive on its inner surface at the interface between said tread and cushion. Said oil rich rubber based adhesive is free, or substantially free, of volatile organic compounds, particularly volatile organic solvents. Alternately, the oil-rich rubber-based adhesive may contain oil and volatile organic solvent wherein oil is the major component insofar as the oil and solvent is concerned.
Historically, a rubber tire is retreaded by first abrading away its tread and buffing a surface of the remaining rubber carcass.
A strip of sulfur vulcanizable, unvulcanized rubber composition is then applied onto and circumferentially around the periphery of the buffed, cured rubber tire carcass.
Such rubber composition strip is conventionally referred as a xe2x80x9ccushionxe2x80x9d.
Over the cushion may then be applied a pre-cured, shaped rubber tread, to which a vulcanizable or unvulcanizable, and preferably a sulfur vulcanizable, unvulcanized rubber-based adhesive has been applied to the inner surface of the tread to enhance the interfacial adhesion between the tread and aforesaid cushion.
The resulting tire assembly is then conventionally encased in a cured butyl rubber envelope, a vacuum drawn on the envelope, and the enveloped tire assembly inserted in an autoclave and the assembly vulcanized therein at a temperature of, for example, about 130xc2x0 C. to produce the retreaded tire.
Such method of tire retreading with a pre-cured tread and vulcanizable cushion over an abraded tire carcass is well known to those having skill in such art.
Historically the aforesaid rubber-based adhesive may be applied to the tread stock as a volatile organic solvent solution of the adhesive and then allowed to dry prior to application of the tread to the aforesaid cushion.
However, it is sometimes desired to provide such adhesive, and the accompanying application of the adhesive to a pre-cured tire tread, in a retreading operation without volatile organic compounds, or at least substantially free of volatile organic compounds, which may sometimes be referred to as xe2x80x9cVOC""sxe2x80x9d.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d as used herein, and according to conventional practice, refers to xe2x80x9cparts of a respective material per 100 parts by weight of rubber elastomerxe2x80x9d. In the description of this invention, the terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d can be used interchangeably, unless otherwise distinguished.
The terms xe2x80x9crubber compositionxe2x80x9d, xe2x80x9ccompounded rubberxe2x80x9d and xe2x80x9crubber compoundxe2x80x9d can be used interchangeably to refer to xe2x80x9crubber which has been blended or mixed with various ingredients and materialsxe2x80x9d and the terms xe2x80x9ccurexe2x80x9d and xe2x80x9cvulcanizexe2x80x9d may also be used interchangeably herein, unless otherwise noted and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In a pneumatic tire retreading operation where a tire carcass is obtained, preferably an abraded vulcanized rubber tire carcass prepared by abrading away its tread and buffing the peripheral surface of the remaining cured tire carcass, to which a strip of a sulfur vulcanizable, unvulcanized rubber composition, or cushion, is circumferentially applied to the abraded peripheral surface of said tire carcass, and to which a pre-cured rubber tread strip, which has a vulcanizable or unvulcanizable, but preferably a sulfur vulcanizable, unvulcanized rubber based adhesive on its inner surface, is applied to said cushion, wherein said rubber based adhesive is located at the interface between said cushion and said tread stock, and wherein the resulting assembly is vulcanized at an elevated temperature to form a retreaded tire; an improvement wherein said adhesive is comprised of an oil rich rubber composition
(i) which contains from about 150 to about 1400, alternately about 400 to about 1000, phr of rubber processing oil and is exclusive of volatile organic compounds; or
(ii) which contains from about 150 to about 1400, alternatively about 400 to about 1000, phr combination of rubber processing oil and volatile organic solvent wherein the weight ratio of said oil to said solvent is in a range of about 1/0.05 to about 1/0.66, preferably about 1/0.05 to about 1/0.33; wherein said solvent is evaporated from said adhesive layer prior to application of said pre-cured tread strip to said cushion layer.
Accordingly, in one aspect of this invention, a retreaded tire is provided as a concentric circular laminate comprised of:
(a) a cured rubber tire casing, or carcass, having an abraded, or buffed, circumferential peripheral surface;
(b) a concentric, circular, unvulcanized rubber cushion layer having an inner surface and outer surface, wherein the inner surface of said cushion layer is positioned on said outer peripheral surface of said tire casing; and
(c) a pre-cured rubber tire tread strip having an inner surface and an outer surface, wherein said inner tread surface contains an unvulcanized oil-rich rubber-based adhesive layer thereon and wherein said pre-cured tread strip is circumferentially positioned on the outer surface of said positioned cushion layer with said adhesive layer therebetween to create a tire assembly, wherein said adhesive layer is comprised of an oil-rich rubber-based composition:
(i) which contains from about 150 to about 1400, alternately about 400 to about 1000, phr of rubber processing oil and is exclusive of volatile organic compounds; or
(ii) which contains from about 150 to about 1400, alternatively about 400 to about 1000, phr combination of rubber processing oil and volatile organic solvent wherein the weight ratio of said oil to said solvent is in a range of about 1/0.05 to about 1/0.66, preferably about 1/0.05 to about 1/0.33; wherein said solvent is evaporated from said adhesive layer prior to application of said pre-cured tread strip to said cushion layer.
A retreaded tire is then provided by heating the said tire assembly to an elevated temperature to vulcanized said adhesive cushion layers.
Usually an elevated temperature in a range of about 100xc2x0 C. to about 150xc2x0 C. is used for such vulcanization.
The oil-rich, rubber-based, adhesive layer is the oil-rich adhesive provided by this invention.
A significant aspect of the invention is the use of a substantial amount of oil which, in the adhesive, is a rubber processing oil and is therefore compatible with the elastomer composition of the cured elastomer of the tread and unvulcanized elastomer composition of the cushion and with the cured elastomer composition of the tire carcass casing to which the tire tread is applied.
This aspect of the invention relies upon the high level of oil content to provide for solubilizing the rubber of the adhesive to permit application of the adhesive to the inner surface of the pre-cure tread and, after sufficient migration of the oil into the cured tread strip, to also provide protection from premature oxidation of the inner tread surface from the atmosphere and impart a tacky surface to the associated surface tread prior to the actual application of the tread to the cushion.
Suitable oils for this adhesive composition are typical rubber processing oils of viscosities (ASTM D 2161, at 99xc2x0 C.) ranging from about 30 to about 70 SUS and preferably 30 to 45 SUS; of clay bed analyses (ASTM D 2007) ranging from about 10 to about 90 wt % aromatics, preferably 20 to 50 wt % aromatics; and saturates ranging from about 90 to about 10 wt % saturates, preferably 80% to 50% saturates; aniline point (ASTM D 611) ranging from about 11 to about 121xc2x0 C. and preferably 15 to 82xc2x0 C.
Where a volatile organic solvent is used in the oil rich adhesive of this invention, representative solvents are, for example, hexane, heptane and mixtures of liquid hydrocarbons containing from 6 to and including 7 carbon atoms. Preferably, such solvents have a boiling point at atmospheric pressure in a range of about 30xc2x0 C. to about 130xc2x0 C.
The adhesive rubber composition may be comprised of various rubbers such as, for example, diene-based rubbers as polymers and copolymers of conjugated diene monomers having from 4 to 10 carbon atoms (e.g. 1,3-butadiene and isoprene), rubbers as copolymers of conjugated diene monomer(s) having from 4 to 10 carbon atoms copolymerized with a vinyl substituted aromatic monomer(s) having from 8 to 12 carbon atoms (e.g., styrene and alpha methylstyrene), and the like, as well as blends thereof. Such rubbers generally contain various antioxidants, fillers such as carbon black, oils, sulfur, accelerators, tackifying resins, antioxidants, stearic acid, antiozonants and the like in conventional amounts.
Representative of such rubbers are, for example, cis 1,4-polyisoprene (natural or synthetic), cis 1,4-polybutadiene and styrene/butadiene copolymer rubber.
Preferably, such rubber is, or is at least primarily comprised of cis 1,4-polyisoprene rubber, preferably natural rubber. For example, such rubber may be natural cis 1,4-polyisoprene rubber or a blend of natural cis 1,4-polyisoprene rubber and at least one of said diene-based rubbers so long as at least 51 weight percent of the rubbers is the natural rubber.
Various rubber reinforcing carbon blacks may be used for the adhesive composition such as, for example, ASTM D 3849 grades N550 and N650 and the like.
The adhesive compositions of this invention can optionally, but preferably, include one or more compatible tackifying agents which are utilized in an effective amount to promote good tack (e.g., pressure sensitive tack) with vulcanized elastomeric substrates, such as the tread or even a buffed carcass surface, if desired, and also with uncured, vulcanizable elastomeric substrates (i.e. non-vulcanized or at least substantially non-vulcanized) such as the said cushion layer. The tackifying resins which can be utilized are generally well known to the art and to the literature and generally include rosin and its derivatives and various hydrocarbon resins. The rosin group comprises rosins, modified rosins and their various derivatives such as esters. The hydrocarbon resin group comprises polyterpines, synthetic hydrocarbon resins, and various modified or special resins which are primarily phenolics. Tackifier resins are described in more detail in the Handbook of Pressure-Sensitive Adhesive Technology, edited by Donatas Satas, Van Nostrand Reinhold Company, 1982, Chapter 16, pages 353-369, which is hereby fully incorporated by reference.
Another and preferred type of tackifier for the adhesive are the various phenol-formaldehyde resins. Such resins generally have a number average molecular weight of 2,000 or less. Typically, alkyl phenols are used rather than phenol itself since the alkyl group improves the miscibility of the resin with the rubber. The existence of phenol-formaldehyde tackifiers are well known to the art and to the literature, e.g., xe2x80x9cResins Used in Rubbersxe2x80x9d by Paul 0. Powers, Rubber Chemistry and Technology, Vol. 36, pages 1542-1562, (1963), and xe2x80x9cRole of Phenolic Tackifiers in Polyisoprene Rubber,xe2x80x9d by F. L. Mangus and G. R. Hamed, Rubber Chemistry and Technology, vol. 64, pages 65-73 (1991).
The present invention relates to a vulcanizable or unvulcanizable system, but the preferable vulcanizable system relates to sulfur cure systems and include one or more cure accelerators in the adhesive composition for this invention. Suitable amounts of sulfur and/or sulfur donor-type compounds generally range from about 1 to about 10 and preferably from about 2 to about 5 phr. The amounts of sulfur vulcanization accelerator generally range from about 0.2 to about 4 and preferably from about 0.5 to about 2.0 phr. Various sulfur accelerators can be used such as aldehyde-amine accelerators, e.g., the reaction product of butyraldehyde and aniline, amines such as hexamethylene tetramine, guanidines such as diphenyl guanidine, thioureas, sulfenamides, thiazoles, and the like. Activators such as zinc oxide, stearic acid, litharge, magnesia and amines can also be used in conventional amounts to attain good crosslinking efficiency.
The oil rich adhesive compositions of the invention can also include conventional amounts of various known rubber compounding ingredients such as processing aids, stabilizers, antidegradants, and the like. Suitable antioxidants include alkylated hindered phenols, alkylated or arylalkylated diphenylamines, aminophenols, dialkyl- diaryl- or mixed alkyl-aryl-p-phenylenediamines, and aniline plus ketone or aldehyde condensation products.
The uncured elastomeric adhesive composition of the present invention can be vulcanized by heat or radiation according to any conventional vulcanization process. Typically, the vulcanization is conducted at a temperature ranging from about 100xc2x0 C. to about 250xc2x0 C. or preferably from about 120xc2x0 C. to about 170xc2x0 C. for a time period ranging from about 1 to about 300 minutes.
The present invention can be utilized to form a retreaded tire, as a laminate of tread, cushion and carcass, for various types of vehicle tires such as passenger car tires, light and medium truck tires, off the road tires, and preferably is utilized in forming retreaded laminates for aircraft and medium truck tires.
To prepare the highly oil rich adhesive compositions, a conventional masterbatch may be first prepared by conventional mixing techniques including, e.g., kneading, roller milling, extruder mixing, internal mixing (such as with a Banbury(copyright) mixer), etc. Such masterbatches may contain conventional amounts of rubber processing oil (0 to about 100 phr oil). The sequence of mixing and temperatures employed are well known to skilled rubber compounders, the objective being the dispersion of fillers, activators, curatives in the polymer without excessive heat buildup.
Additional rubber processing oil may then be incorporated into the masterbatch in mixers designed for lower viscosity materials such as paints or caulks. Such mixers include those with high speed dispersion blades such as a churn.
Curatives may be incorporated into the conventional masterbatch, or during the subsequent addition of the extra oil.